bootloader: Calculate SHA256 hash of image on every boot

bootloader: Calculate SHA256 hash of image on every boot

Makes app image booting more reliable (256-bit rather than 8-bit verification.)

Some measurements, time to boot a 655KB app.bin file and run to app_main() execution.

(All for rev 1 silicon, ie no 340ms spurious WDT delay.)

80MHz QIO mode:
before = 300ms
after = 140ms

40MHz DIO mode:
before = 712ms
after = 577ms

40MHz DIO mode, secure boot enabled
before = 1380ms
after = 934ms

(Secure boot involves two ECC signature verifications (partition table, app) that take approx 300ms each with 80MHz CPU.)